1. Field of the Invention
The invention relates to a method and apparatus for controlling centrifugal and axial compressor surge to enhance equipment/personnel safety and to minimize energy consumption.
2. Description of the Prior Art
Conventional surge control techniques for centrifugal and axial compressors provide for surge estimation based on the polytropic head (Hp) ratio to the volumetric suction flow rate squared (Qs2). Such conventional surge control techniques are disclosed and described in publications. These conventional methods set the surge limit line of the compressor based on the surge curve data calculated by the compressor manufacturer. The conventional methods do not take process changes and compressor efficiency decreases into account. Thus the actual compressor surge limits usually differ significantly from the limits indicated on the compressor surge curve data calculated by the Compressor Manufacturer.
Microprocessor-based controllers with an anti-surge control algorithm have been used for compressor incipient surge detection. There are existing systems in the field where compressor control is handled by a distributed control system. Most compressor controls executed with distributed control systems consist of oversimplified algorithms. The main reason for this inefficient control is the speed of executing the algorithm and control system itself. Control systems without the special algorithm and speed requirements have used external devices that required special mounting and separate power wiring.
Before a compressor reaches the actual surge point rapid oscillations of flow, pressure, and current occur. Compressor field tests have confirmed this phenomena as an indication of impending surge. This invention is to provide a method and apparatus to detect incipient (impending) surge based on special conditioning of the high-speed oscillation measurement(s).
The invention relates to a method and apparatus for continuously monitoring the high frequency of a process variable signed, such as flow or pressure or driver motor current oscillations before the compressor reaches the actual surge point. This incipient surge control acts as an override control to the primary surge control and, in addition to allowing for optimum surge point setting, increases compressor/personnel safety while widening the operating window and saving energy.
The present invention uses a very simple but highly effective means of detecting an incipient surge of a compressor. Unique characterization and filtering is required to distinguish the surge characteristics of the compressor from the normal operation characteristics.
The accurate determination of the true surge control line not only eliminates the risk of encountering a surge condition, but also minimizes unnecessary wide surge margins which can result in excessive recycle/blowoff and waste of energy.
A special, high speed algorithm is required to transform pre-surge oscillations into useful data for control purposes. The detecting is accomplished by separating the high-frequency oscillation part of a process signal and transmitting the amplitude of this high-frequency oscillation component of the system signal via a controlled current loop to a controller. The circuit is housed in a small standard rail mounted enclosure.